1. Field of the invention
The invention relates to terrain navigation and, more particularly, to estimations of curvilinear distance taking into account any potential obstacles to be circumvented.
2. Description of the prior art
The Applicant has already proposed, in a French patent application filed on Sep. 26, 2003, under the number 0311320, a method for estimating the distances, with respect to a reference point, of the points on a map taken from a database of elevation of the terrain. This method implements a distance transform by propagation and takes account of obstacles to be circumvented whose shape may change over time, such as mountains seen by an aircraft having an imposed vertical flight profile with altitude variations which mean that the same ground feature posing a threat at a certain moment no longer poses a threat at another or vice versa. The distance transform by propagation is used to establish a distance map covering a region of a given size where a craft is traveling and having the instantaneous position of the craft as origin of the distance measurements. This distance map, which is not designed to be displayed, is used to trace a geographical map which is displayed and which represents the region of travel segmented into areas shown in false colors which are bounded as a function of the possibility for the craft to get past them and of the time that the latter would take to reach them when they are passable, for example red for impassable obstacles, yellow for far away accessible regions and green for nearby accessible regions.
While the estimations of distance made when the distance map is established take into account the obstacles to be circumvented and the change in the threat that they represent for the craft as a function of the degree of accomplishment of its mission, they do not take into account the maneuverability of the craft so that the distance estimations made lack realism for certain points situated in the near neighborhood of the craft. In fact, especially in the case of an aircraft, the estimations of distance are conducted in the same fashion whether the point considered is located in front of or behind the aircraft, even though the aircraft can reach a point located in front of it without difficulty but must make a complete U-turn in order to reach a point located behind it, the extent of the U-turn depending on its capacity to turn, in the direction in which it steers: with the wind or against the wind, and on the strength of the cross-wind.